Submitted by: @pavel-zotov

1)
echo create database 'localhost/3400:C:\MIX\firebird\QA\fbt-repo\tmp\e40.fdb';|C:\MIX\firebird\fb40\isql -q -z

ISQL Version: WI-T4.0.0.420 Firebird 4.0 Unstable
Server version:
WI-T4.0.0.420 Firebird 4.0 Unstable
WI-T4.0.0.420 Firebird 4.0 Unstable/tcp (csprog)/P14
WI-T4.0.0.420 Firebird 4.0 Unstable/tcp (csprog)/P14

2)
C:\MIX\firebird\fb40\gfix -z -b 3000 localhost/3400:C:\MIX\firebird\QA\fbt-repo\tmp\E40.FDB

gfix version WI-T4.0.0.420 Firebird 4.0 Unstable
Error reading data from the connection.

firebird.log:

CSPROG Thu Oct 27 23:59:44 2016
INET/inet_error: read errno = 10054, server host = localhost, address = 127.0.0.1/3400

CSPROG Thu Oct 27 23:59:45 2016
setsockopt: error setting IPV6_V6ONLY to 0

The same result if run:

C:\MIX\firebird\fb30\fbsvcmgr localhost/3400:service_mgr action_properties prp_page_buffers 3000 dbname C:\MIX\firebird\QA\fbt-repo\tmp\E40.FDB

PS.
No such trouble on 4.0 SuperClassic.
No such trouble on 3.0 SuperServer.